In the operation of conventional cathode ray tubes (CRT's), such for example as those utilized in television receivers and monitors, an image is created by generating one (for monochrome images) or a plurality (for color tubes) of modulated electron beams and continuously sweeping the beams across an imaging screen to thereby selectively excite the phosphors or other coatings on the screen. Scanning movement of the beam(s) about the imaging screen is effected by the application of time-varying control voltages to horizontal and vertical deflection coils or yokes. Beam focusing is carried out by applying suitable control voltages to one or, more typically, to a plurality of generally electrostatic lens elements. In color CRT's, convergence of the plural beams at a shadow mask or other screen grid disposed closely proximate the imaging screen is controlled by time-varying voltages applied to electrostatic deflection electrodes or to electromagnetic convergence coils, depending on the tube design.
Each of these electron beam controlling devices in a cathode ray tube directly affects the quality of the images generated on the imaging screen by operatively adjusting one or more parameters or affecting one or more characteristics of the image. The focusing or lens elements directly control the sharpness or definition of the image. The convergence electrodes or coils, by appropriately deflecting each of the plural electron beams to a dynamically-moving common point at the shadow mask, assure that each beam excites its respective color phosphor pixels and, therefore, provide proper color registration in the resulting image. And the deflection coils--particularly though not exclusively the horizontal sweep or deflection coils--must continuously move the electron beam(s) across the imaging screen at a constant linear velocity to avoid geometric distortion of the screen-generated images.
One of the major difficulties inherent in maintaining such image quality-affecting aspects as proper electron beam focus, convergence and sweep linearity is the constantly changing path length of the beam as it scans across the imaging screen. As is well understood by those skilled in the art the distance from the electron beam focusing lens elements to the imaging screen is constantly changing during operation of the tube and is significantly greater at the peripheral extremes of the screen than at its center. This dynamically-changing path length requires constant adjustment of, for example, the focal length of the beam(s), the convergence bias on the beams in a color tube, and the linear velocity of the vertical and, in particular, of the horizontal deflection sweeps. Although it is known, for this purpose, to apply time-varying correction voltages to these components of a CRT, such adjustments are typically based on the calculated, anticipated positions of the scanning beam--derived for example as a function of time and set, once, at the time of manufacture or initial tube alignment--rather than on the sensed, actual position of the beam as it sweeps across the imaging screen. Moreover, such currently practiced adjustments--sometimes referred to as digital dynamic focus and convergence--provide only gross corrections over relatively large areal zones, as for example defined by a 16 by 16 matrix over the entire imaging screen. Neither do heretofore known arrangements provide compensation for the effects of component aging or the like, or for more than relatively small deviations in focus and convergence, other than through the ability to effect periodic manual adjustments which, typically, must be performed by skilled service personnel. And no currently known CRT imaging systems of this type can or do provide closed loop, real time correction of focus, or convergence, or beam sweep linearity, or any other parameters that affect the quality or clarity of images produced on the imaging screen of the tube.